Unvulcanized cis-1,4-polyisoprene elastomers, both natural and synthetic, particularly the synthetic elastomer, typically do not have sufficiently desirable green strength in order to permit assembling, or building, various rubber composition-based components which contain such elastomers to form articles of manufacture such, as for example, tires.
A typical major deficiency of unvulcanized synthetic cis-1,4-polyisoprene, and therefore various rubber compositions which contain unvulcanized synthetic cis-1,4-polyisoprene is a usual lack of sufficient green strength and tack needed for satisfactory processing or building properties required in the building of articles of manufacture including the building of tires. The abatement of such usual deficiency has often been sought and may assist in facilitating a replacement, or at least a partial replacement, of natural rubber for appropriate rubber compositions.
The term "green strength", while being commonly employed and generally understood by persons skilled in the rubber industry, is nevertheless a difficult property to precisely define. Basically, green strength may be thought of as the tensile strength developed when an unvulcanized polymer composition of proper configuration is stressed under controlled conditions. Beyond an initial yield point, unvulcanized natural cis-1,4-polyisoprene rubber compositions will show increasing strength against rupture, or significant deformation, while unvulcanized synthetic cis-1,4-polyisoprene will typically fall below the yield point or will increase only slightly above it. In certain practical applications such as uncured tires, belting, shoes and a number of other products in the course of manufacture, green strength is important in promoting the integrity and cohesiveness, including dimensional stability, of the assembly of various rubber components between building or assembly thereof and the ultimate molding and accompanying vulcanization of the assembled article.
Green strength often manifests itself secondarily in the tack or adhesiveness imparted to various unvulcanized rubber compositions employed in the manufacture of a number of rubber articles such as tires, belting, etc. Other things being equal, an unvulcanized rubber or rubber composition having higher green strength will often exhibit better building tack or adhesion to other unvulcanized rubber-based components and will accordingly ease various fabrication, processing and handling problems associated with the building and the ultimate molding and vulcanization of fabricated articles.
Various additive compounds or agents which have heretofore been utilized to improve green strength of synthetic rubber elastomers, for example, numerous nitroso compounds as mentioned in U.S. Pat. Nos. 2,457,331, 2,477,015, 2,518,576, 2,526,504, 2,540,596, 2,690,780 and 3,093,614. Additionally, various compounds have been mentioned such as those described in U.S. Pat. Nos. 2,969,341, 3,037,954, 3,160,595 and British Patent No. 896,309. Yet another class of additives or compounds are the diesters of 5-norbonene as mentioned in U.S. Pat. Nos. 3,817,883 and 3,843,613. In U.S. Pat. No. 4,124,750 a dihydrazide compound is suggested for cross-linking a synthetic rubber to enhance its green strength.
In the description of this invention, the term "phr" is used to designate parts by weight of a material per 100 parts by weight of elastomer. In the further description, the terms "rubber" and "elastomer" may be used interchangeably unless otherwise mentioned. The terms "vulcanized" and "cured" may be used interchangeably, as well as "unvulcanized" or "uncured", unless otherwise indicated.